There is growing concern that adolescence is associated with greater sensitivity to psychostimulant drugs and increased risk of addiction. Such increased susceptibility to drug self-administration could be due to differences in the way the adolescent brain reacts or adapts to repeated drug exposure compared with the adult brain. The overall goal of this proposal is to understand the neurophysiological and behavioral determinants that contribute to adolescent susceptibility to cocaine self-administration, using rodent models. The dopamine system will be the focus of these studies, as it plays a central role in mediating the rewarding effects of addictive drugs. The specific aims are: to determine whether adolescence is characterized by heightened excitability of dopamine cells, and whether this is mediated by decreased intrinsic inhibitory properties of these neurons (Aim 1) and/or increased excitatory synaptic input to these cells (Aim 2). Aim 3 will establish whether changes in dopamine neuron function translate into differences in cocaine self-administration behavior. Throughout these aims, adolescent and adult rats will be compared both before and after repeated cocaine exposure to appraise whether individuals exhibit inherent differences, and/or show differential cocaine- induced neuroadaptations that could facilitate self-administration behavior. Neuronal activity will be assessed using in vivo and in vitro electrophysiology techniques;behavioral tests will consist of a variety of self-administration procedures that allow estimating the rewarding effects of cocaine and motivation to self-administer the drug. Overall, using a multidisciplinary approach this proposal will examine, from the cellular to the behavioral level, potential mechanisms underlying adolescent/adult differences in cocaine self-administration. These studies will use behavioral models that are relevant to the human condition to provide insight into both the causes and effects of adolescent drug use. In the long run, these findings will facilitate the development of new targeted therapeutic strategies for the treatment or prevention of cocaine addiction.